Everybody in the US seems to be sold out, Marstar in Canada lists them but you will need import paperwork filed and approved. $20 Canadian. The PPS43 mags look like they would work with a slight mod, they fit into the magwell fine but you would need to Dremel a bit on them to get the mag catch to hold them in. Don't have a working example of an M56 handy so can't verify if they would feed OK, but it looks like it would work.

Have 4 'afternarket'yugo m56 mags usual(Eastern block) HDheavy gauge steel man,strange small squareof sheet metal spot welded on both sides towads top of mag.3in exc 4th may work but previous owner, decesed friend im disposing of this stuff for may have used a flat-bladed sctrewdriver as a punch? Fancied hiself a smith!lol pay shipping from 53357 and $25.